1. Technical Field
The present invention relates to a method for producing a multi-viewpoint image for three-dimensional image display which acquires a multi-viewpoint image for three-dimensional image display.
2. Background Art
As a three-dimensional image displaying method, there is a horizontal parallax system where a flat display typified by a liquid crystal display (LCD) where pixels are arranged on a two-dimensional plane in a matrix form and an optical plate such as a lenticular sheet (a ridge line extends in a vertical direction as viewed from a viewer) for controlling a light ray direction or a slit (the same as the above) are combined and an image corresponding to a horizontal position in a viewing (observing) direction is viewed (observed) so that a three-dimensional image is recognized. The horizontal parallax system includes sub-classes of an integral imaging system, a multi-view system, and a binocular system which are the same regarding the fact that parallax is given in a horizontal direction.
Multi-viewpoint images acquired from a plurality of viewpoints different from one another in a horizontal direction are required for three-dimensional image display in the horizontal parallax systems. That is, a three-dimensional image is approximately reproduced by approximately reproducing images acquired from different viewpoints in a horizontal direction at different viewing positions (viewpoints) in the horizontal direction.
According to each system, when viewing ranges for a three-dimensional image are different and acquiring positions and the numbers of multi-viewpoint images regarding a display plane are different but a horizontal direction is defined as X direction, a vertical direction is defined as Y direction, and a viewing direction and a multi-viewpoint image acquiring direction are defined as Z direction, only X component of coordinates for a plurality of viewpoints fluctuates at equal intervals. This is due to that pixels on the flat display are arranged at equal intervals. Projection faces of images acquired from respective viewpoints are caused to coincide with a display plane of a three-dimensional image display apparatus (hereinafter, also called “3D display”) and a target viewpoint is caused to coincide with the center of the display.
However, when x coordinates for respective viewpoints are made different while fixing the target viewpoint, angles defined by straight lines connecting viewpoints and the target viewpoint and a projection plane to be caused to coincide with a display plane of the original 3D display are different for respective viewpoints. When such image acquisition is realized by using a camera in an actual space or a camera model producing an image utilizing computer graphic (CG), it is necessary to use lens shifting function.
However, currently, most of cameras and camera models do not have the function. When an image is acquired utilizing a camera or a camera model which does not include a lens shifting function, an angle defined by a projection plane and a straight line which connects a viewpoint and a target viewpoint is fixed to 90°. That is, projection planes of images acquired from respective viewpoints are eventually different for the respective viewpoints. Keystone correction is required to correct deviation of a projection plane of images acquired from the respective viewpoints and an ideal projection plane to be caused to coincide with a display plane on a 3D display from each other.
As a method where the keystone correction is not conducted, there is known a method performed using a camera or a camera model which does not have a lens shifting function, where target viewpoints (individual target viewpoints) are prepared for respective viewpoints, images are acquired while maintaining angles defined by lines connecting viewpoints and individual target viewpoints and a projection plane at 90°, and respective viewpoint images are cut from overlapping regions of the projection plane shifted by deviations of individual target viewpoints (see Japanese Patent Laid Open Publication No. 2003-43413).
On the other hand, in a three-dimensional image displaying method of the horizontal parallax system, it is known that there is a display limitation in a jumping-out direction and in a depth direction (see H. Hoshino et al., J. Opt. Soc. Am. A., 15(8), 2059 (1998)). More specifically, as far as the resolution of a flat panel display is constants the resolution of the 3D display, the range where a three-dimensional image can be viewed (observed), and the jumping-out and depth limitation are put in tradeoff relationship. The jumping-out and depth limitation is limited to several centimeters to several tens centimeters in view of the resolution and the size of a current flat panel display. When a display plane of a 3D display is set to be vertical like an ordinary television set or a monitor for a personal computer, there occurs such a problem that an object to be displayed is limited in view of the range of the display limitation. For example, a distant view or an object just before a viewer, or an object having a thickness of ± several tens centimeters can not be displayed.
An object put on a flat surface tends to be limited regarding its height due to its stability, as compared with a floor area. That is, when an area of the display is limited, a height of an object displayed thereon is also limited necessarily.
As the feature of the horizontal parallax system 3D display, there is a problem that an immovable point to a display screen frame is fixed on a screen, where when a person observes in a horizontal direction, he/she thinks that infinity is an immovable point unconsciously. That is, even if an observation object appears to move leftward and rightward by swinging his/her head, he/she recognizes that the immovable point set at infinity and the object do not move and he/she himself/herself has moved.
However, in the three-dimensional display of a horizontal parallax system, recognition is made such that an immovable point is present on a display plane such as a display casing or an edge of a print object of a printing matter so that an object on this side appears to move in a direction opposed to a moving direction of an viewer while an object at a depth side appears to move in the same direction as the moving direction of the viewer. That is, a distant view appears such that it moves largely in the same direction as an observer. This causes such a problem that discrepancy from a real space is impressed.
As a similar phenomenon in the real space, it is assumed that scenery is viewed through a window. The scenery is not viewed utilizing the center of the window as a reference (an immovable point). On the other hand, when an object on a horizontal plane is viewed down, a person sets the immovable point on a plane naturally. That is, even if there is a target viewpoint (an immovable point) on a display (or a print object in a printing matter), uncomfortable feeling does not occur at all.
When a display of the horizontal parallax system is put in a flatbed manner, since there is not any parallax information in a vertical direction, it is known that natural display can be realized by acquiring respective viewpoint images to have angles to a normal line to a display plane to display a three-dimensional image (Japanese Patent Laid Open Publication No. 2003-43413). Hereinafter, such a display method is called “photographing with depression”.
As described above, however, since an angle defined by a line connecting the viewpoint and the target viewpoint and a projection plane is deviated from an angle of 90° due to the depression like the case that the lens shifting function in a horizontal direction (in X direction) is required when a multi-viewpoint image is acquired, a lens shifting function in a vertical direction (in Y direction) is required. As a method for correcting this deviation, the keystone correction is adopted in the technique described in Japanese Patent Laid Open Publication No. 2004-198971.
However, it is difficult to perform keystone correction of two axes extending in the horizontal direction for acquiring a multi-viewpoint image and in the vertical direction for realizing the depression and an image with higher resolution must be acquired so as to allow enlargement and reduction for correction.
As described above, in acquisition of a multi-viewpoint image for a three-dimensional image display of a flatbed horizontal parallax system, a photographing method equivalent to the lens shifting function of two axes of a horizontal direction and a vertical direction is required, but a camera or a camera model having a lens shifting function is restrictive, and such a photographing method can not be said to be an ordinary method. The keystone correction of two axes itself is higher in difficulty than the keystone correction of one axis and the former causes distortion in an image easily.